1. Field of the Invention
The invention relates to a process for producing electrical circuits, and more particularly to a lamination process for producing multilayer printed circuit boards with fine pitch surface features.
2. Description of the Related Art
Current multi-chip circuitry requires the attachment of numerous integrated circuit chips to multilayer substrates or boards such as copper-polymide interconnects. The chips are attached to surface mount pads located on the top surface of the substrate. In order to increase the number of chips in a given area, it is important to decrease the pitch of the surface features including the traces and the pads.
Conventional processes for producing multilayer printed circuits employ copper foils for plating bases which are usually at least 5 microns thick. The copper foil is laminated to a substrate and then through-holes are drilled in the substrate. Next, spaced surface features are formed on the foil either additively or subtractively.
In the additive process photoresist is applied and developed on the copper foil. Metal is plated on the exposed portions of the copper foil. The remaining resist is stripped and the underlaying copper foil is removed.
In the subtractive process metal is first plated on the exposed copper and then the resist is patterned on the metal. The exposed metal is removed as is the underlaying copper. Finally, the resist is stripped.
Fine pitch features (less than approximately 6 mil pitch) are difficult to realize, however, with conventional additive and subtractive foil-based printed circuit board fabrication techniques due to feature undercut during etchback. Undercut results from the tendency of wet chemical etchants to etch laterally as well as vertically. The undercut may cause significant amounts of material to be lost, producing deformed features with poor mechanical and/or electrical characteristics.
Several solutions to the copper foil undercutting problem have been proposed. These solutions are usually based on the theme that as the copper foil or film gets thinner, the needed exposure to wet chemical etching and subsequent undercut are reduced. For example, U.S. Pat. Nos. 4,431,710 and 4,499,152 disclose laminates with thin copper films (less than about 16 microns thick). These copper films are vapor deposited on aluminum carrier sheets, a bonding layer is adhesively situated between the copper film and a substrate, and the carrier sheet is peeled away from the copper film. A drawback to this technique is the need for providing and peeling away the carrier sheet. Likewise, U.S. Pat. Nos. 4,705,592 and 4,521,280 reduce undercutting during etching by replacing the traditional copper foil with a thin conductive layer (about 80 to 500 nanometers thick) which is sputtered onto a roughened top surface and into through-holes of a substrate in a single step. While this produces a thin plating layer and may reduce costs by eliminating the need for depositing a plating seed in the through-holes, it is still relatively expensive since the sputtering must be performed on a small surface after lamination has occurred. Furthermore, precatalyzed substrates are known in the art.